Diaphragm and blades for turbomachinery

ABSTRACT

A diaphragm for an axial flow turbomachine, in which outer shrouds of adjacent fixed blades in a row of blades contact each other circumferentially to form a circumferentially continuous load path. Inner shrouds of the blades only contact each other on contact faces oriented to transmit loads in the radial and/or axial directions of the turbomachine.

FIELD

Exemplary embodiments relate to the arrangement of turbine blades toform a turbine diaphragm of fixed blades that can operate at hightemperatures and that results in a reduction in working fluid leakage inturbines caused by distortion of the blade rows due to changes in theoperating temperature.

BACKGROUND

Steam turbines convert the energy in steam firstly into mechanicalenergy, in the form of rotational energy, and then into electricalenergy. Multiple rows, which are termed stages, of turbine blades areused to rotate a turbine shaft. Each steam turbine stage alternatelyconsists of stationary and rotating components: the stationarycomponents are rows of turbine blades mounted to the inside of thecasing of the turbine, and are herein referred to as ‘fixed blades’; andthe rotating components are rows of turbine blades mounted to a turbinerotor, and are herein referred to as ‘moving blades’.

The pressurised steam enters the turbine axially and first impinges onthe blade surfaces of a row of fixed blades. The blades deflect thesteam onto a row of moving blades which in turn also deflect the steamback to the axial direction, causing themselves move in the oppositedirection to the deflected steam. This causes the turbine rotor torotate and the steam to expand slightly. The next stage of fixed andmoving blades, repeats the process. This process continues through theturbine until the steam is completely expanded.

Each successive stage of blades is optimised to deal with the pressureand volume of the steam expected at the blades' location in the turbine,as the steam will become successively less pressurised as it movesthrough the successive rows of turbine blades.

As shown in FIGS. 1 and 2, the fixed turbine blades 103, 203 can bemounted either directly in the turbine casing 100, 200 or in separatediaphragms 202. The blades making up a turbine stage are interconnectedto provide damping, thereby avoiding possible vibrations which coulddamage the turbine.

Referring to FIG. 1, there are small axial clearances between the fixedblades 103 and the moving blades 105 to prevent the blades contactingeach other. There are also small radial clearances between the fixedcasing 100 and the rotating components 105, 108; and between the rotor101 and the stationary components 103, 109. These clearances must bemade as small as possible to avoid steam leakage, as steam flow throughthe clearances does not pass through the blading and so is unable toproduce any power. Sealing fins 104 are provided in the radialclearances to reduce the amount of steam passing through them. Thesealing fins 104 may be fixed either to the rotor 101, to the casing 100or to the ends of the blades 103, 105.

In the case where the fixed blades 103 are mounted in the casing 100, asshown in FIG. 1, any distortion of the casing 100 due to thermal effectswill affect the radial clearance between the ends of the blades 109 andthe rotor 101 as the row of blades 103 will no longer form an accuratecircle. This can result in some of the ends of the turbine blades 109contacting the sealing fins 104, whilst the rotor 101 is rotating, andas a result the sealing fins 104 becoming damaged. Once the distortionsin the casing 100 disappear, this damage to the sealing fins 104 leadsto increased leakage of the steam, because the sealing fins 104 are lessable to inhibit steam leaking through the radial clearance between theend of the turbine blade 109 and the rotor 101.

To protect the fixed blades, and therefore the sealing fins 104, fromthe above distortion of the casing, without having to increase theradial clearances between the ends of the blades and the rotor, thefixed blades can be mounted in a diaphragm as shown in FIG. 2. Thediaphragm 202, 203, 204 is usually a welded structure, in two halves toallow it to fit around the turbine shaft, with an outer 202 or inner 204ring with sufficient mass to ensure that radial distortion is minimisedand the blades 203 thus remain in an accurate circle. The outer ring ofthe diaphragm 202 is mounted to the inner surface of turbine casing 200in a groove 201, and the inner ring of the diaphragm 204 fits within agroove 205 in the rotor 207. The inner ring of the diaphragm 204 doesnot contact the rotor 207, creating a clearance in-between, but a finnedseal 206 is provided in the groove 205 in the rotor 207 to reduce steamflow through the clearance. The moving blades 209 are positioned axiallyadjacent to the fixed blades 203 provided in the diaphragm 202, 203, 204and are fixed to the rotor by a moving blade root 208. At the end of themoving blades 209 there is provided a moving blade shroud portion 210,creating a clearance between the moving blade shroud portion 210 and theturbine casing 200. This clearance is likely to be provided with anotherfinned seal to reduce steam flow through the clearance.

However, recent designs of diaphragm are much more compact, as shown inFIGS. 3, 4 and 5. In the arrangement shown in FIG. 3, the fixed blades303 are mounted in a compact diaphragm 302, 303, 309 with an outer ring302 and an inner ring 309. Seals 306B are provided to reduce steam flowthrough the clearance between the inner ring of the diaphragm 309 andthe rotor 301. The moving blades 304 have blade roots 305 mounted in therotor 301. Seals 306A are provided in the clearance between the outershroud 307 of the moving blade 304 and the inside surface of an axiallyprojecting portion 310 of the outer ring of the diaphragm 302. Theaxially projecting portion 310 of the outer ring of the diaphragm 302lies radially between the turbine casing 300 and the outer ring of thediaphragm 302.

This design of diaphragm allows for advantageous rotor construction,such as allowing the use of a drum rotor and t-root fixings. However,this means that the thermal inertias of the outer ring 401, 402 andinner ring 405, 406 of the diaphragm 400, 500 shown in FIGS. 4 and 5differ. The result of this is that the outer 401, 402 and inner rings405, 406 heat up, and cool down, at different speeds to each other.

As shown in FIG. 4, the outer and inner rings of the diaphragm 400 mustbe split at 403, 407 into two halves, so splitting the diaphragm acrossits diameter, to allow it to be positioned around a rotor. The differingthermal expansion resulting from the difference in temperatures cancause the two halves of the diaphragm to distort as shown in exaggeratedform in FIG. 5, so that together they form a figure of 8 or oval shape.This means that in some regions of the circumference the stationaryparts move closer to the moving parts, closing up the clearance betweenthem which can then cause damage when the fins contact the blades or therotor, resulting in a permanent increase in leakage as described above.

An exemplary purpose of the invention is, therefore, to reduce oreliminate the problem of a compact diaphragm containing a row of turbineblades suffering thermal distortion which results in increased steamleakage and damage to the turbine.

SUMMARY

In brief, the invention provides a turbine diaphragm for an axial flowturbomachine, in which outer shrouds of adjacent fixed blades contacteach other circumferentially to form a circumferentially continuous loadpath, but in which inner shrouds of the blades only contact each otheron contact faces oriented to transmit loads in the radial and/or axialdirections. This arrangement can avoid circumferential load pathsthrough the inner shrouds and thereby ameliorates the stated problem ofthermal distortion.

To achieve this result consistently, an interference fit can be providedbetween adjacent inner shrouds on their contact faces, and theinterference fit can apply sufficient torque forces to the shrouds toensure that the contact faces remain in contact with each otherthroughout the temperature range of operation of the turbomachine.

In a preferred exemplary embodiment, the contact faces for transmittingloads in radial directions contact each other when the diaphragm is inthe as-assembled cool condition and throughout all operating conditionsof the turbine, but the contact faces for transmitting loads in axialdirections only contact each other when the diaphragm reaches anoperating temperature.

Opposed side edges of the inner shrouds contact corresponding side edgesof adjacent inner shrouds of adjacent blades and each opposed side edgecomprises a projecting step portion, a recessed step portion and achamfered step portion that joins the projecting step portion to therecessed step portion, the projecting step portions being at oppositeends of their respective side edges and configured to project intoco-operating recessed step portions of adjacent inner shrouds ofadjacent blades, the chamfered step portions comprising contact facesoperative to transmit loads in axial directions between adjacent innershroud portions and to prevent circumferential transmission of loadsbetween adjacent inner shroud portions.

Preferably, each opposed side edge of the inner shroud portion furthercomprises a planar portion, the projecting step portions comprise partsof the side edges that jut out relative to the planar portions, and therecessed step portions comprise parts of the side edges that areundercut relative to the planar portions. To transfer radial forcesbetween adjacent inner shroud portions, it is arranged that contactfaces of the planar portions, the projecting step portions and therecessed step portions radially abut each other.

In another aspect, exemplary embodiments provides a blade for use in arow of fixed blades in an axial flow turbomachine, comprising:

(a) a radially outer shroud portion,

(b) a blade aerofoil portion, and

(c) a radially inner shroud portion having two opposed side edges forcontacting corresponding side edges of adjacent inner shroud portions ofadjacent blades in a row of such blades,

wherein each opposed side edge comprises a projecting step portion, arecessed step portion and a chamfered step portion that joins theprojecting step portion to the recessed step portion, the projectingstep portions being at opposite ends of their respective side edges andconfigured to project into co-operating recessed step portions ofadjacent inner shroud portions of adjacent blades, the chamfered stepportions being arranged to transfer forces between adjacent inner shroudportions transversely of the circumferential direction in the row ofblades and to prevent circumferential transmission of loads betweenadjacent inner shroud portions.

An exemplary turbine blade can interconnect on its inner edge withneighbouring blades, but not transmit circumferential tensile andcompressive forces to these neighbouring blades. This can beaccomplished by an arrangement that ensures each blade remains free toexpand in the circumferential direction whilst keeping contact betweenthe blades. With a small circumferential clearance, for example of lessthan 0.5 mm, neighbouring blades no longer transmit the tensile orcompressive forces that cause the diaphragm to distort under heating orcooling. The blades are held in position through fixing to the outerring of the diaphragm.

Further aspects of the invention will be apparent from a perusal of thefollowing description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described, withreference to the accompanying drawings, in which like reference symbolsindicate the same or similar components.

FIG. 1 is a partial section taken in a radial plane coincident with therotational axis of a turbine showing the arrangement of a fixed blademounted in the casing and a moving blade mounted in the rotor;

FIG. 2 is a partial section taken in a radial plane coincident with therotational axis of a turbine showing the arrangement of a fixed blademounted in a massive diaphragm and a moving blade mounted in the rotor;

FIG. 3 is a partial section taken in a radial plane coincident with therotational axis of a turbine showing the arrangement of a fixed blademounted in a compact diaphragm and a moving blade mounted in the rotor;

FIG. 4 is an end view along the turbine's rotational axis showing a rowof fixed blades mounted in a compact diaphragm seen in isolation fromother turbine structure;

FIG. 5 is a view similar to FIG. 4, but showing, in exaggerated form,the row of fixed blades undergoing distortions caused by inner and outerrings of the diaphragm being at different temperatures due to thedifferent thermal inertias of the inner and outer rings;

FIG. 6 is a perspective view of three neighbouring turbine bladesaccording to the preferred embodiment of the invention;

FIGS. 7 a to 7 c are perspective views of a turbine blade according tothe preferred embodiment of the invention, each view being on adifferent side of the blade;

FIG. 8 a is a perspective view of the three neighbouring turbine bladesof FIG. 6 being mounted on an outer ring of the diaphragm;

FIG. 8 b is a partial section taken on line B-B of FIG. 8 a showing theouter shroud portion of one of the turbine blades contacting the innersurface of the diaphragm's outer ring before welding has occurred;

FIGS. 9 a and 9 b are enlarged cross-sectional views of a stepped edgejoint on the inner shroud portions of the neighbouring turbine blades ofFIG. 6, showing the joint before undergoing heating (FIG. 9 a) and afterundergoing heating (FIG. 9 b);

FIG. 10 is a view similar to FIG. 9 b, showing the forces acting on thejoint when a full set of turbine blades are inserted into a turbinediaphragm; and

FIG. 11 is a perspective view of a turbine blade according to analternative embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A turbine blade according to the preferred embodiment and a diaphragmcontaining a row of such turbine blades, will now be described withreference to FIGS. 6 to 10.

FIGS. 7 a and 7 b show a single turbine blade 700 according to thepreferred embodiment of the invention. The blade 700 is formed as asingle solid part, forged and machined from a metal block, in threeportions: an outer shroud portion 710, a blade portion 720 and an innershroud portion 730.

The outer shroud portion 710 is formed as a substantially rectangular orparallelogram-shaped plate having four edge faces 711, 712, 714, 716. Itis curved in the circumferential direction of the turbine diaphragm sothat when all the blades are assembled into the diaphragm, adjoiningouter shroud portions 710 form a ring whose centre of curvaturecoincides with the turbine's rotational axis.

The radially inner surface 713 of the outer shroud portion 710 forms aflow surface of the turbine passage. During manufacture of the turbinediapragm, the radially outer surface 715 of the outer shroud portion 710is fixed by welding to an inwardly projecting flange 805 of an outerring 800 of the diaphragm, as shown in FIGS. 8 a and 8 b and describedlater.

Circumferentially facing edges 712, 714 of the outer shroud portion 710extend generally in the axial direction of the turbine and aresubstantially planar surfaces. When the blades 700 are assembled intothe diaphragm, there is circumferential contact between the shroud edges712, 714 of neighbouring shrouds to form a circumferentially continuousload path, but as explained below there is no circumferential contactbetween the inner shroud portions 730. The axially facing,circumferentially extending edges 711, 716 of the outer shroud portion710 are also substantially planar surfaces and the distance between themis the same axial width as the outer ring 800.

The blade portion 720 comprises an aerofoil 721 that connects the outershroud portion 710 to the inner shroud portion 730. The inner shroudportion 730 is formed as a substantially rectangular orparallelogram-shaped plate that is curved in the circumferentialdirection of the turbine diaphragm so that when all the blades areassembled into the diaphragm, adjacent inner shroud portions 730 form aring whose centre of curvature coincides with the turbine's rotationalaxis. In the assembled turbine diaphragm, the radially outer surface 740of the outer shroud portion 710 forms a flow surface of the turbinepassage and the radially inner surface 738 seals against the rotor,e.g., by means of sealing fins mounted on the rotor, similar to fins 306in FIG. 3.

Like the outer shroud portion 710, the inner shroud portion 730 hassubstantially planar axially facing, circumferentially extending edges741, 742. However, unlike outer shroud portion 710, eachcircumferentially facing, generally axially extending edge of the innershroud portion 730 has a projecting step portion 732, 734, acomplementary recessed step portion 736, 737, a chamfered step portion743, 744 that joins the projecting step portion to the recessed stepportion, and a planar portion 731, 733. The planar portions 731, 733occupy half the height of their shroud edges, extend the full axialextent of the inner shroud 730 and are located radially outward of therecessed and projecting step portions. The projecting step portions 732,734 comprise parts of the shroud edges that jut out relative to theplanar portions 731, 733 of the shroud edges, whereas the recessed stepportions 736, 737 comprise parts of the shroud edges that are undercutrelative to the planar portions 731, 733. Half the axial extent of theinner shrouds is occupied by the projecting step portions 732, 734,which occupy axially opposed positions on the opposed circumferentiallyfacing edges of the inner shroud. Similarly, the recessed step portionsextend over the remaining half of the axial extent of the inner shroudsand occupy axially opposed positions on their respective shroud edges.Hence, when the blades are assembled into the turbine diaphragm, theprojecting step portions 732, 734 of each inner shroud mate with therecessed step portions 736, 737 of the neighbouring inner shrouds, toform sliding differential expansion joints between the inner shrouds, asexplained in more detail below.

The sliding differential expansion joints between neighbouring innershrouds 730 have contact faces comprising the radially outward facingsurfaces 735 of the projecting step portions 732, 734, the radiallyinward facing surfaces 735 ¹ of the recessed step portions 736, 737(which may also be characterised as overhanging surfaces of the planaredge portions 731, 733), and the surfaces of chamfered step portions743, 744 that form angled faces between the recessed and projecting stepportions. Hence, when the turbine diaphragm is in the fully assembledcondition, the contact faces 735 on any given inner shroud 730 radiallyabut the contact faces 735 ¹ on the neighbouring inner shrouds totransmit radial loads between the inner shrouds. Furthermore, atoperational temperatures of the turbine, the chamfered step portions743, 744 on any given inner shroud 730 also abut each other to transmitloads between the inner shrouds transversely of the circumferentialdirection, i.e., in a generally axial direction. However, thecircumferential facing surfaces 733, 734, 736; 731, 732, 737 of theinner shroud portions 730 do not contact each other, but remainseparated by a small gap of about 0.1 mm to 0.5 mm to prevent thetransmission of tensile or compressive forces in the circumferential ortangential direction. Transmission of these forces in thecircumferential direction, as mentioned previously, would lead to thediaphragm being pulled out of axi-symmetry around the rotor, with theconsequences mentioned in relation to the prior art. Therefore, theabove-mentioned small circumferential gap is left between neighbouringinner shroud portions 730 to allow for thermal expansion.

It should be understood that in this embodiment, the design is such thatwhen a full row, or stage, of blades 700 is assembled as a diaphragm,internal twisting forces are produced by flexing the aerofoil portion721 of the blade during insertion of the blade into the diaphragm. It isarranged that in the as-assembled cool condition, the internal twistingforces cause the abutting contact faces 735, 735 ¹ to be forcedtogether. As every abutting contact face has the same amount of internaltwisting force acting on it, the net force is zero when all theneighbouring inner shrouds are in mating contact.

To recap, the inner shroud portion 730 of the blade 700 is adapted tointerlock with the neighbouring inner shroud portions without the innershroud portions coming into contact in the circumferential/tangentialdirection, i.e., there is no appreciable load transmission between theinner shrouds in directions perpendicular to a plane coincident with theturbine axis.

To construct a turbine diaphragm containing a row or stage of fixedblades 700 for incorporation in a turbine, the blades 700 are insertedinto a T-shaped outer ring 800. The radially outer faces 715 of theouter shrouds 710 abut the radially inner face 808 of a radiallyinwardly projecting flange 805 that forms the stem of the T-shaped outerring 800. The abutment of the outer shroud portions 710 and the flange805 creates two nominally cylindrical channels 804 between the mainportion 801 of the outer ring 800 and the interconnected outer shroudportions 710 of the blades 700. To secure the blades 700 within theouter ring 800, a welding head is inserted into the channels 804 and theouter shrouds are fillet welded to the flange 805 in an automatedwelding process, as known.

Once the diaphragm is constructed as detailed above, it is cut acrossits diameter at the outer ring 800 into two semicircular sections. Theouter shrouds 710 of the blades 700 are not fixed to each other, so theouter ring 800 is cut at a point where two outer shrouds meet. Thisallows the two parts of the diaphragm to be placed around the rotor inthe turbine when the turbine is being assembled. The two semicircularsections of the outer ring 800 can then be secured together again, e.g.,by means of inserting strong bolts through pre-existing bolting flangesof the outer ring 800, as known, causing the complete circumferentialload path in the outer shrouds to be restored.

With reference to FIGS. 7 and 10, the forces acting on two neighbouringinner shrouds 730 when the diaphragm is assembled will now be furtherdescribed. As already mentioned, during assembly of the blades into thediaphragm, the aerofoils 721 are twisted slightly out of their naturalalignment with respect to the outer shroud portions, with the resultthat the inner shroud portions are forced into contact with each otheron contact faces 735, 735 ¹. As seen in FIG. 10, a projecting stepportion 734 of the right hand inner shroud 730 projects into aco-operating recessed step portion 737 of the left hand inner shroud,with the radially inward facing contact face 735 ¹, formed by therecessed step portion 737 abutting the radially outward facing contactface 735 of the projecting step portion 734. Similarly, a projectingstep portion 732 (FIG. 7 c) of the left hand inner shroud projects intoco-operating recessed step portion 736 (FIG. 7 b) of the right handinner shroud, with the radially inward facing contacting face 735 ¹,formed by the recessed step portion 736, abutting the radially outwardfacing contact face 735, formed by the projecting step portion 732.Equal and opposite forces F act radially at the abutting contact faces735, 735 ¹, creating a zero net force when the entire row of blades 700is assembled. In essence, when assembled into the turbine diaphragm,there is an interference fit between adjacent shrouds on their radialcontact faces 735, 735 ¹. This applies sufficient torque to the shroudsto ensure that the contact faces 735, 735 ¹ remain in hard contact witheach other during operation of the turbine.

It should be understood that in the as-assembled condition, when theturbine is not operating and the blades 700 are at ambient temperature,the chamfered step portions 743, 744 do not contact each other. This isbecause, as shown in FIG. 9 a, the gap between the planar portions 731,733 of neighbouring shroud edges is relatively wide. However, onheating, the shrouds expand such that the projecting step portions 734,735 extend further into the respective co-operating recessed stepportions 736, 737, until the faces of the chamfered step portions 743,744 come into contact with each other. This prevents the projecting stepportions 732, 734 from projecting all the way into the recessed stepportions 736, 737 and preserves a small inter-shroud gap as shown inFIG. 9 b, to ensure there is no circumferential load path through theinner shrouds. Further thermal expansion of the inner shrouds generatesequal and opposite forces on the abutting chamfered contact faces 743,744, which contribute a zero net force in the assembled operatingturbine diaphragm. The forces at the chamfered step portions acttransversely of the circumferential/tangential direction.

In an alternative non-preferred embodiment shown in FIG. 11, theexpansion joint mechanism between the inner shroud portions 730 a ofneighbouring blades 700 a differs from that shown in FIGS. 6 to 10. Inthe preferred embodiment as described in relation to FIGS. 6 to 10, thecontact faces 735, 735 ¹ of the inner shrouds, when assembled, contacteach other in a radial direction relative to the axis of the turbine,but the chamfered step portions 743, 744 only contact each other whenthe turbine reaches an operating temperature. However, in thealternative embodiment of FIG. 11, the radial contact faces are omittedand interference contact between the inner shrouds 730 a in theas-assembled cool condition of the diaphragm occurs on the faces of thechamfered step portions 743 ¹ and 744 ¹ of the inner shroud edges,leaving a small circumferential gap between the inner shroud edges ofadjacent blades, as was the case in the preferred embodiment. This againcan avoid transferring forces between inner shrouds 730 a in thecircumferential direction because the chamfered step portions 7431, 744¹ react loads in a generally axial direction.

As can be seen, the circumferentially facing edges 731 a, 733 a of theinner shroud portion 730 a are each provided with a projecting stepportion 733 a occupying substantially half the axial length of eachcircumferentially facing edge, the projecting step portions being ataxially opposite ends of their respective circumferential facing edges.Each chamfered step portion 743 ¹, 744 ¹ forms an angled face between arecessed step portion 731 a and the projecting step portion 733 a. Theseangled faces of the chamfered step portions 743, 744 contact the facesof the chamfered step portions of neighbouring inner shrouds insubstantially axially abutting relationship when the turbine diaphragmis assembled.

Although the above description mentions welding of the outer shrouds 710to the outer ring 801, other ways of connecting blades 700 to the outerring of the diaphragm are available, such as through a T-root type offixing, or similar.

In still further embodiments the expansion joint mechanism can be usedin other types of turbines, such as gas turbines. Furthermore, theinvention could also be applicable to fixed blades in compressors.

Exemplary embodiments can be used over a wide range of temperatures andpressures experienced by turbines, e.g., 150 to 600 degrees Celsius and5 to 300 bars. Steel and/or nickel alloys or other appropriate materialscan be used in the fabrication of the turbine components described here.

The present invention has been described above purely by way of example,and modifications can be made within the scope of the invention asclaimed. The invention also consists in any individual featuresdescribed or implicit herein or shown or implicit in the drawings or anycombination of any such features or any generalisation of any suchfeatures or combination, which extends to equivalents thereof. Thus, thebreadth and scope of the present invention should not be limited by anyof the above-described exemplary embodiments. Each feature disclosed inthe specification, including the claims and drawings, may be replaced byalternative features serving the same, equivalent or similar purposes,unless expressly stated otherwise.

Any discussion of the prior art throughout the specification is not anadmission that such prior art is widely known or forms part of thecommon general knowledge in the field.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise”, “comprising”, and thelike, are to be construed in an inclusive as opposed to an exclusive orexhaustive sense; that is to say, in the sense of “including, but notlimited to”.

1. A blade for use in a row of fixed blades in an axial flowturbomachine, comprising: (a) a radially outer shroud portion, (b) ablade aerofoil portion, and (c) a radially inner shroud portion havingtwo opposed side edges for contacting corresponding side edges ofadjacent inner shroud portions of adjacent blades in a row of suchblades, wherein each opposed side edge comprises a projecting stepportion, a recessed step portion and a chamfered step portion that joinsthe projecting step portion to the recessed step portion, the projectingstep portions being at opposite ends of their respective side edges andconfigured to project into co-operating recessed step portions ofadjacent inner shroud portions of adjacent blades, the chamfered stepportions being arranged to transfer forces between adjacent inner shroudportions transversely of the circumferential direction in the row ofblades and to prevent circumferential transmission of loads betweenadjacent inner shroud portions.
 2. A blade according to claim 1, whereineach opposed side edge of the inner shroud portion further comprises aplanar portion, and wherein the projecting step portions comprise partsof the side edges that jut out relative to the planar portions, and therecessed step portions comprise parts of the side edges that areundercut relative to the planar portions.
 3. A blade according to claim2, wherein contact faces of the planar portions, the projecting stepportions and the recessed step portions are arranged to radially abuteach other in the row of blades, thereby to transfer radial forcesbetween adjacent inner shroud portions.
 4. A blade according to claim 1,the blade being a steam turbine blade.
 5. A blade according to claim 1,the blade being a gas turbine blade or a compressor blade.
 6. Adiaphragm for an axial flow turbomachine, comprising: outer shrouds ofadjacent fixed blades in a row of blades which contact each othercircumferentially to form a circumferentially continuous load path; andinner shrouds of the blades which only contact each other on contactfaces oriented to transmit loads in a radial and/or axial direction ofthe turbomachine.
 7. A diaphragm according to claim 6, in which there isan interference fit between adjacent inner shrouds on their contactfaces, and the interference fit applies sufficient torque forces to theshrouds to ensure that the contact faces remain in contact with eachother throughout operation of the turbomachine.
 8. A diaphragm accordingto claim 6, wherein the contact faces for transmitting loads in radialdirections contact each other when the diaphragm is in the as-assembledcool condition and throughout all operating conditions of the turbine,but the contact faces for transmitting loads in axial directions onlycontact each other when the diaphragm reaches an operating temperature.9. A diaphragm according to claim 6, wherein opposed side edges of theinner shrouds contact corresponding side edges of adjacent inner shroudsof adjacent blades and each opposed side edge comprises a projectingstep portion, a recessed step portion and a chamfered step portion thatjoins the projecting step portion to the recessed step portion, theprojecting step portions being at opposite ends of their respective sideedges and configured to project into co-operating recessed step portionsof adjacent inner shrouds of adjacent blades, the chamfered stepportions comprising contact faces operative to transmit loads in axialdirections between adjacent inner shroud portions and to preventcircumferential transmission of loads between adjacent inner shroudportions.
 10. A diaphragm according to claim 9, wherein each opposedside edge of the inner shroud portion further comprises a planarportion, and wherein the projecting step portions comprise parts of theside edges that jut out relative to the planar portions, and therecessed step portions comprise parts of the side edges that areundercut relative to the planar portions.
 11. A diaphragm according toclaim 10, wherein contact faces of the planar portions, the projectingstep portions and the recessed step portions are arranged to radiallyabut each other, thereby to transfer radial forces between adjacentinner shroud portions.
 12. A steam turbine diaphragm according to claim6, in combination with a steam turbine.
 13. A diaphragm according toclaim 6, in combination with a gas turbine or compressor.
 14. A turbineincluding a diaphragm according to claim
 6. 15. A compressor including adiaphragm according to claim 6.